Lateral buckling of beams - The Behaviour and Design of Steel Structures to EC3 - page 249

Shear diaphragm

Shear diaphragm

I-beam

M M

L

(a) Elevation

(b) Section

Figure 6.16 Diaphragm-braced I-beams.

and the greatest value of this is

α L = 4 M cr

Ld f .

(6.46)

The flange force Q f at elastic buckling can be approximated by

Q f = M cr / d f , (6.47)

and so the minimum top-flange translational stiffness can be approximated by

α L = 4 Q f / L . (6.48)

This is of the same form as equation 3.33 for the minimum stiffness of
intermediate restraints for compression members.
There are no restraint stiffness requirements in EC3. This follows the finding
[25, 26] that compression member restraints which are capable of transmitting
2.5%oftheforceinthecompressionmemberinvariablyarestiffenoughtoensure
the second mode buckling. Thus EC3 generally requires any restraining element
at a plastic hinge location to be capable of transmitting 2.5% of the flange force
in the beam being restrained.
Theinfluenceofintermediaterestraintsonbeamswithcentralconcentratedand
uniformlydistributedloadshasalsobeenstudied,andmanyvaluesoftheminimum
restraintstiffnessesrequiredtocausethebeamstobuckleasifrigidlybracedhave
been determined [16, 24, 27-30].
The effects of diaphragm bracing on the lateral buckling of simply supported
beams with equal and opposite end moments (see Figure 6.16) have also been
investigated[16,31,32],andasimplemethodofdeterminingwhetheradiaphragm
is capable of providing full bracing has been developed [31].